[英]Swift cast protocol type to struct and pass as inout
protocol ValueHolder {
}
struct A: ValueHolder {
var value = 5
}
var x: ValueHolder = A()
func f(a: inout A) {
a.value = 10
}
I want to use pass x to f. 我想将x传递给f。 Is it possible?
可能吗?
Edit: I understand all of the staff about value semantics and inout parameters. 编辑:我了解所有有关值语义和inout参数的人员。 The problem is that
x
needs to be casted to A
and I wonder if that can be done without copying it ( as
makes a typed copy of it) so that I can use f
on x
. 问题是
x
需要强制转换为A
,我想知道是否可以不复制而将其复制( as
对它的类型化复制),以便可以在x
上使用f
。
I want to use pass x (and not its copy) to f.
我想使用传递x(而不是它的副本)到f。 Is it possible?
可能吗?
Not exactly — at least, not if the parameter is an A. An A is a struct. 不完全是-至少,如果参数是A,则不是。A是结构。 Even with
inout
, a struct is still a value type. 即使使用
inout
,结构仍然是值类型。 inout
allows the original value to be replaced, but what it is replaced with is another A. Simply put, a struct cannot be mutated in place. inout
允许替换原始值,但替换为另一个A。简单地说,无法在适当位置更改结构。
That being so, your question seems to lose its meaning. 既然如此,您的问题似乎失去了意义。 Since a struct cannot be mutated in place, there is no real reason to use
inout
here at all. 由于结构无法在适当位置进行突变,因此根本没有真正的理由在这里使用
inout
。 You are not doing anything that calls for inout
. 您没有做任何需要
inout
事情。 You might as well just drop the inout
and accept the value semantics: 您不妨删除
inout
并接受值语义:
func f(a: A) -> A {
var a = a
a.value = 10
return a
}
x = f(a:x as! A)
If you really want to keep the inout
, then type a:
as a ValueHolder and cast inside the function, like this: 如果您确实要保留
inout
,则输入a:
作为ValueHolder并在函数内部进行强制转换,如下所示:
var x: ValueHolder = A()
func f(a: inout ValueHolder) {
var b = a as! A
b.value = 10
a = b
}
f(a:&x)
(x as! A).value // 10
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